A civil war is a conflict between two or more groups within a single country, often resulting in significant loss of life and property, as well as long-lasting social and political changes. Civil wars are typically fought over control of the government, ethnic or religious differences, and conflicting ideologies. They can involve large-scale military operations and result in widespread destruction, as well as long-term political instability and economic hardship.
Source: Fearon, J. D. (1995). Rationalist explanations for war. International Organization, 49(3), 379-414.